Out-of-synchronism relay



Patented Sept. 8, 1936 UNlTiE $TATES PATENT OFFICE assignors to Westinghouse Electric &

Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 17, 1934, Serial No. 740,252

19 Claims.

Our invention relates to means for automatically responding to .out-of-synchronism conditions, or low-voltage conditions, in a tie-line joining synchronous transmission networks, and it relates more particularly to means for automatically interrupting the circuit of the tie-line under such conditions.

In many transmission systems, where major amounts of power are transmitted over a transmission line, it is important to discriminate between a fault involving all phase-wires, which requires the interruption of the line circuit, and out-of-synchronism conditions, which are accompanied, at times, by current and voltage conditions simulating a fault involving all phase-wires on the protected line-section, but which should not result in an interruption or the line circuit because such interruption would mean a serious service-interruption because of the loss of power. In other words, in such major-power systems, it is best to let the system operate with parts thereof out-of-synchronism with each other, for a while, while the whole is being brought into synchronism again, rather than permitting the fault-responsive protective apparatus to divide the system into a number of small sections here and there.

Our invention relates, however, to another type of transmission system, in which the tie-line to be protected normally transmits only a relatively .small amount of power across the circuitinterrupting point therein, being used, for example, for the purpose of permitting additional generators, as needed, to be added first at one end of the tie-line and then at the other, instead of requiring an additional generator to be added simultaneously at both ends in order to handle the local load. In such systems, the loss of the tie-line would not cause a serious serviceinterruption, and usually no service-interruption at all, and moreover, the tie-line is usually too weak to bring the two ends of the system into synchronism, anyway, if they should pull apart. Under such conditions, it is the usual practice to interrupt the circuit of the tie-line as promptly as possible, as soon as the existence of out-of- .synchronism conditions is known to the stationattendant.

It is a principal object of our invention to perform this tie-line sectionalizing operation automatically.

It is a further object of our invention, where more than one tie-line is involved, to avoid tieline sectionalization of all of the tie lines under fault conditions affecting less than all of them.

It is a further object of our invention to provide fault-responsive protection at the gener ator stations, utilizing a promptly acting, selective, so-called primary protective means, and a retarded-action, less selective, so-called secondary protective means, at each generating station, while the out-of-synchronism protective means near the mid-point of the tie-line is set to be operative in a time intermediate between the primary and secondary fault-responsive protective means at the generator stations, for reasons that Will be more fully explained hereinafter.

It is a still further object of our invention to automatically cut in the respective primary and secondary protective means only in response to certain predetermined fault conditions on the connected transmission lines, as will be explained hereinafter.

It is a still further object of our invention to automatically recalibrate a fault-responsive protective means in direct response to the number of generators in service at any time.

With the foregoing and other objects in View, as will be explained more in detail in the description which follows, our invention consists in the circuits, systems and operations hereinafter described and claimed and illustrated in the accompanying drawings, wherein:

Figure 1 is a single-line diagram of a system embodying our invention;

Figs. 2 and 3 are curves showing the relations of voltage and current during certain out-of-synchronism conditions;

Fig. 4 is a diagrammatic view of the protective equipment at one end and at the intermediate sectionalizing point of a tie-line protect-ed in accordance with our invention;

Figs. 5 and 6 are diagrammatic views showing modifications; and

Figs. 7 and 8 are curves illustrating the operation of the apparatus shown in Fig. 6.

In the particular application of our invention which is shown diagrammatically in Fig. 1, there is a l32-kilovolt, 25-cycle transmission system embodying parallel circuits or lines I and 2 for supplying power to an electrified railroad embodying a system of 12 kilovolt trolleys 3 which are energized from the transmission lines I and 2 through step-down transformers 4 at various points along the right-of-way. The 25-cycle transmission system is supplied with power at a plurality of points, including two stations L and R, at which are located generator-buses 5 and 6, which are connected to the transmission lines I and 2 through step-up transformers I. In the particular system shown, the generator buses 5 and 6 are supplied with energy from 25-cycle synchronous generators, most or all of which consist of frequency-changers 8 which derive power from a larger 60-cycle transmission network, symbolized by the GO-cycle synchronous generators 9 and i and the 60-cycle tie-line ll between the stations L and R. a V

In such a system, when the relatively large 60- cycle transmission network pulls apart between the stations L and R, the 25-cycle system which is synchronously coupled thereto at both ends will generally also pull apart, because there is, in general, much more generating capacity connected to the 60-cycle system than to the 25-cycle system. The speed at which the 25-cycle system pulls apart, or pulls out-of-step, is quite variable, depending upon the speed with which the 60- cycle'system pulls out-of-step, and this depends upon the amount of load carried over the 60- cycle tie-line H, and other factors.

, 'It is necessary, therefore, to provide means for responding to the out-of-synchronism condition,

both in the case of fast pull-out and in the case of slow pull-out of the 25-cyc1e tie-lines l and 2 between the stations L and R.

'When' two generating points on the system pull out-of-step with each other, the voltage on the 7 system at a point which is somewhere near midway, electrically, between the internal generator Voltages, falls nearly to zero and rises tofull value again. This cycle of operation is repeated each time one power-source or generator slips a pair of poles with respect to'the other. If the point at which the tie-line is to be sectionalized, in the event of out-of-synchronism operation, is near the electrical center of the reactance joining the internal voltages of the twogenerators, thevoltage of the sectionalizing point drops to a very low value each time the generator voltages pass through the position of 180 electrical phase displacement, this Voltage becoming substantially zero at the electrical center of the system, and

- reaching higher and higher minimum values at toring togenerating, and the cycle thereafter re-' points further and further removed from the electrical center.

Figs. 2 and 3 show how the magnitudes of the voltage and current may vary during out-of-synchronism conditions. It is to be noted that these figures do not show the instantaneous directions of the sinusoidal wave-forms of the alternating voltage and current, but show how the voltage and current fluctuate in their root-mean-squarev magnitude, that is, the scaler values of the voltage and current, without reference to the phaserelations thereof. 7

Normal load-conditions are depicted at H3 in Figs. 2 and 3. When the'two generating stations start to drift apart, the voltage atsome intermediate point of the tie-line drops in value, until it reaches a minimum value M, Fig. 2, when the internal generator-voltages are 180 apart, with maximum current i5, Fig. 3. The voltage therea after rises, and the current decreases, until the generator voltages come in phase with each other, at 360, at which point the voltage is again a maximum, as indicated at it, and the current is a minimum, as indicated at H. During this half of the out-of-synchronism cycle, however, the voltage between the points it and it builds up in the opposite direction than it was at first, representing a condition of reverse power-flow. At the point ll, as the generator voltages pass through 360, the current reverses, and changes from mopeats itself, three successive cycles being illus trated in Figs. 2 and 3. No particular efiort has been made, in these figures, to indicate accurately either the magnitudes or the accelerationsinvolved in the pulling-apart of the two generators;

it is merely intended to illustrate the general principles.

The time-scale, along the X-axis, has not been indicated in Figs. 2 and 3. The time for the completion of three cycles of out-of-synchronism pulsations may be of the order of one to live seconds, with fast pull-out, or it may require that long,.or longer, fora single cycle of out-ofsynchronism pulsation in the event of a 'slow pull-out.

Referring again to Fig. 1, it will be noted that there is an intermediate station D on the25-cycle tie-line I, 2 somewhere near the electrical center,

of the 25-cycle system, and in case of necessity, the interruption of the tie-line circuit is efiected at this intermediate station D by means of circuit interrupters 2H and 22, respectively, in the tie- 23 in the tions. The particular protective'means, and the V particular times of operation, indicated in the present specification, are, of course, illustrative only, as various'means may be adopted for the purpose.

The primary protective means at the station L is shown, in Fig. 4, as comprising a difierential current relay 26, receiving power from currenttransformers 27 and 28 in the lines I and 2, re-

spectively. This relay 26 closes either one of its contacts 29 and .30, respectively, in response to predetermined conditions of unbalance, according as the line I or the line 2 carries the heavier current. It is'designed to operate in a relatively quick time, which may be from .07 to .7 second,

although we are not necessarily. limited. to this particular time of operation, except that it is faster in its operation than the secondary pro.-

tection means which will be described later on. 1

the line l or the line 2, as the case may be, which v is at fault, as indicated by the heavier currentfiow therein;

This prmary protection means is intended to i be quite selectively responsive to predetermined fault conditions, so that it will trip one, of the breakers 2G or25 only in case of an actual fault which requires the tripping of that breaker in order to clear the fault, care being taken to avoid the unnecessary tripping of either one of the circuit breakers 20 or '25. To this end, we preferably utilize a fault-detector in the shape of under-voltage relays 3i and 32, energized from the lines l and 2 through capacitor voltage-couplers 33 and 35, and having make-contacts 35, 35 and 3?, 38 whichshort-circuit the operating coils 39 and d0 of the difierential current relay 26 mechanism,

age, appears -on;both lines 4 and 2. The faultdetector 3 |-32 thus makes it possible to give the difierentialv current relay a sensitive setting, by keeping current out of the relay-coils 39-49 under normal operating-voltage conditions and preventingtripping on load-unbalanoes.

The secondary protection means at the station Liis illustrated, in Fig. 4, as comprising a pair of relatively slow over-current relays 4| and 42 which are energized from the current-transformers 21 and 28, respectively, and which close their contacts 43 and 44,in response to predetermined over-currents in the lines I and 2, in a time which is longer than'the maximum time of operation of the primary protection means 26. This described operation of the over-current relays 4| and 42 may be inherent, in any manner, in the relay design, such as by having sulficient inertias so that it takes a certain necessary time in order to move the movable contact element far enough to close a contact, or'by means of a dashpot which is indicated, at 45, for the sake of illustration, in Fig. 4. The closure of the over-current relay contacts 43 and 44 results in the tripping of the circuit breakers 24 and 25, respectively, as will be readly understood from the drawings.

In accordance with our invention, we have provided the over-current secondary-protection means 4| and 42 withan automatic recalibrating involving variable-tap autotransformers 46 and 41 which are utilized to connect the operating coils of the over-current relays 4| and 42 to the current-transformers 21 and 28, respectively. The taps on the autotransformers 46 and 41 are changed. by movable contact-members 49 and 50 carried by a movable contact-rod 5| which may be reciprocated by means of a reversible pilot-motor 52 controlled by means of a reversing-relay 53.

A distinctively novel feature of our recalibrating mechanism is that it responds automatically to the connection and disconnection of the variour generators on the generator-bus 5 so that the settings of the over-current relays 4| and 42 are made larger, in the proper amount, when more generators are connected in service, and vice versa; Thus, we have shown three frequency-changers 8 at station L, having three synchronous generators 54, 55, 56, which are connected to the generator-bus 5 through circuitbreakers 51, 58, 59.

The reversing relay 53 for controlling the pilotmotor 52 is shown as a differential relay of the balance-beam type, having two opposed operating-coils 6| and 62. One of the operating-coils, for example, the coil 6|, is constantly energized by means of a circuit including a resistor 63. The energizing-circuit of the other relay-coil 62 contains five small automatically controlled resistor-elements 54, 55, 66, 61 and 68, each having half the resistance of the resistor 63. The resistors 64 and B5 are cut in or out, according to the position of the autotransformer tap-changer rod 5|, by means of a movable contact-member 69 thereon. The resistors 66, 61, and 68 are cut in or out, according to the positions of the generator circuit-breakers 51, 58, 59, by means of auxiliary switches 1|, 12 and 13 carried thereby.

Thus, we obtain automatic recalibration of the secondary-protection over-current relays 4| and 42, as follows. When all of the generators 54, 55 and 55 are in service, the three resistors 66, 61 and 68 are short-circuited by the auxiliary breaker-switches 1|, 12 and 13, and the two resistors 64 'and'65 are in circuit with the operating coil 62 of the reversing relay 53, thus balancing the double resistor 63, which is permanently in circuit with the other coil 6| of the reversing relay. Under these conditions, the parts are in the position shown in Fig. 4, with all of each of the autotransiormers 45 and 41 in shunt with the operating coils of the overcurrent relays 4| and 42, respectively, thus causing the secondary currents of the current-transformers 21 and 28 to flow, ful1strength, through the over-current relays 4| and 42. In this position, the tap-changer rod is at its extreme lefthand limit of travel, further movement being safeguarded against by a limit-switch 14.

When one of the generators, say 55, is disconnected, its associated resistor 68 is connected in circuit with the reversing-relay coil 62, thus weakening the energization of the latter, so that the coil 6| pulls the relay over to the left and energizes the pilot-motor 52 in the direction necessary to move the tap-changing rod 5| to the right. When the contact-members 49 and 53 have moved over, one tap, on the autotransformers 46 and 41, the contact-member 69 will also have moved over, so as to short-circuit the resistor 55, thus restoring the reversing relay 53 to its balanced neutral position, and deenergizing the pilot-motor 52. The efiect of the autotransformers 43 and 41 is now to cause the overcurrent relays 4| and 42 to receive more current than is flowing in the secondary circuits of the respective current-transformers 21 and 28, thus causing the over-current relays to reach their settings at lower values of the line-currents.

When two of the generators 54, 55, and 5B are disconnected from service, the tap-changer rod 5! is moved over, one more tap, and it reaches its extreme right-hand limit of travel, where further movement is prevented by a limit-switch 15. When all three generators are disconnected, the energization of the reversing-switch coil 62 is weakened by the inclusion or" three resistors, 65, 61, and 68, in its circuit, thus canting the movable element of the reversing switch to the left, but the pilot-motor 52 is not energized because of the opening of the limit-switch 15.

The over-current relays 4| and 42 of the secondary protection means are preferably also controlled by a fault-detertor which may, or may not, have the same setting at the fault-detector which controls the primary protection means 26. As illustrated, the under-voltage relays 3| and 32, are provided with additional contacts and 11 which short-circuit the operating coils of the respective over-current relays 4i and 42 whenever the voltages on the respective lines I and 2 are above a predetermined value.

The protective equipment at the intermediate sectionaliaing station D of the tie-lines and 2 of the -cycle system includes means for automatically opening the sectionalizing breakers 2|, 2'2, and 23 in the event of out-of-synchronism operation. As previously indicated, we have provided out-of-synchronism relays for both fast and slow pull-apart.

The relays for fast pull-apart are a pair of so-called voltage-dip relays 8| and 82 which are illustrated as being energized from capacitor voltage-couplers 83 and 84 associated with the lines and 2, respectively. These voltage-dip relays 8i and 82 pick up, or drop out, as the linevoltage rises above, or falls below, the relay-setting. These relays have front-contacts 85 and back-contacts 86, which are made and opened, respectively, when the relay picks up.

In order to provide a sufficient number of contacts, of sufiicient size to have the necessary current-carrying capacity, each of the voltage-dip relays SI and 82 is illustrated as being associated .with an auxiliary relay 9I and 92, respectively,

the operation of which is controlled by the voltage-dip relays (H and 82, so that the additional or heavy contact members may be placed on the auxiliary relays BI and 92 rather than on the sensitive voltage relays 8! and 82. The operating coils of the auxiliary relays 0| and 92 are shortcircuited by the back-contacts 86 of the voltage relays 8| and 82, respectively, when the voltage relays are deenergized or insufliciently energized. As. soon as these voltage relays pick up, their back-contacts 86 are opened, and a circuit is completed from the positive relaying bus, through the front-contacts 85 of'the relays Bl and 82, and thence through the operating coils of the auxiliary relays SI and 92, respectively, to the negative relaying bus. 7 V

The relays 9I and 92 are shown also as completing a holding-circuit through front-contacts 93. When the voltage-dip relays 8I and 82 drop out, the above-mentioned energizing circuits of the auxiliary relays 9i and 92 are deenergized, including also the holding-circuits through the front-contacts 93, said holding-circuits being deenergized by the short-circuiting of the operating coils of the auxiliary relays BI and 92 which occurs when the back-contacts 80 of the voltage relays'8I and 82 are closed. a

The auxiliary Voltage relays 9I and 92 are provided with back-contacts 94 and 95, respectively, which are connected in series with each other, and which are interposed between an auxiliary positive bus 99 and the operating coil of a counter-controlling relay I06 which controls the operation of a counting-chain for counting the number of times the line-voltage dips and comes back to normal. It will be noted that this counter-controllingrelay I00 is energized only when both lines I and 2 have less than normal voltage thereon, and is deenergized when either line I or 2 has normal voltage thereon. This arrangement is made so that a dip in voltage on only one of the lines, ora complete deenergization of one line, does not start the counting-chain.

When the counter-controlling relay I00 is energized in response to the first voltage-dip, its trout-contact I BI completes a circuit from the auxiliary positive bus 09 to the operating coil of a vibrator relay I02 which has a vibrating reed I03 carrying a weight I0 3 on its end, which causes it to continue to vibrate for 3 to 5 seconds after an impulse has been given thereto, and released again, the, time of vibration being adjustable within said limits. As long as this reed continues Vibrating, it alternately makes frontand backcontacts I05 and I00, which are utilized to complete a circuit from the auxiliary positive bus 99, through the reed I03, to a counting-chain timerrelay I 07 of the retarded type, as indicated by the lag-ring I08 which so opposes any sudden change in flux therein as to make the relay quite slow in its drop-out movement, although it picks up quite rapidly because of the exceedingly 7 strong magnetizing force which is provided by the first-mentioned auxiliary positive 'bus 99.

This second auxiliary positive bus III is continued on, to a third auxiliary positive bus I12 which is normally connected to the second bus III but which may be disconnected therefrom,

through means which will'be described later, on. The energization of the third bus II2 immediately energizes a first counting-chain relay I I3, whichpicks up and closes its front-contact IM.

It will be well to consider,.a moment, the con dition of the apparatusat this stageof the operation. On the first voltage-dip on both trans mission lines'I and 2, the counter-controlling relay I00 had picked up, and had energized the vibratory relay I 02, holding it. steady with its reed against the front-contact I05, thereby energizing the timer I02. On the first restoration of line-voltage, the vibrating relay I02 is deenergized, thereby causing the vibrating reed I03 20" during the periods between successive coniple- 25- tions of the circuit through the alternating front and back-contacts I05 and I00 of the vibrating reed I03, until the amplitude of the vibrations is insufiicient to keep the contact closed long enough to supply the necessary holding-energy for the 30 timer relay.- Then, in order to avoid a short period of chattering oi the timer-relay I01, as the vibrating relay is making its last slight contacts at I05 and 506, the timer-relay contact I00 is provided, so as to open up when the timer relay 35 first drops out, so that thereafter, if exceedingly brief contacts are made by the vibrating reed at I05 and I05, the-timer-relay I07 receives energy from only the front-contact I05, and it immediately suffers a sudden reduction in the amount of energy which it receives, due to the cutting'out of the back-contact I00 of the vibrating relay, thereby causing the timer-relay I07 to open positively and stay openthroughout the'remainder of the faint contacts of the dying vibrations of the reed I03. I We will now go back to the point in the opera tion, when the line-voltage was first restored, after the firstvoltage-dip. Under these conditions, the counter-controlling relay I 00 is deenergized. a

In its deenergized position, the counter-controlling relay I0i closes a back-contact I I5 which extends a circuit from the third auxiliary positive bus IE2, through the relay-contact M4, to a second counting-chain relay, IIB, which picks up and holds itself in, through a front-contact In which by-passes the relay contacts 'I I4 and H5, so that the relay IISremains in its picked-up positionas long as the third auxiliarypositive bus I 52 is energized. The second counting-chain relay I It has a back-contact E 58 which is in series with the operating coil of the vibrating relay I02, so that the energization of thersecond countingchain relay H6 in response to the first restoration of line-voltage locks out the vibrating relay I 02, so that" the latter, is not reenergized even when the front-contact IQI of the counter-coniliarybuses III and H2 are necessarily deenergized through the front-contact I ID of the timer relay IO'I.

The second counting-chain relay I I6 is further provided with a front-contact III! which is in series with a front-contact I29 of the countercontrolling relay IIlil, so that, when the second voltage-dip occurs, the counter-controlling relay I00 will again pick up and extend a circuit from the third auxiliary positive bus IIZ, through the relay-contacts H9 and I20, to a third countingchain relay I2 I When this relay I2I picks up on the second voltage-dip, it completes a holding circuit, by means of its front-contact I22, around the relay-contacts H9 and I23, and it also closes a front-contact I23 which is in series with a backcontact I24 on the counter-controlling relay I22.

If the voltage of either tie line I or 2 rises a second time above its predetermined minimum normal value, determined by the setting of the voltage-dip relays 8! and 82, the counter-controlling relay IEiQ will again drop out, closing its back-contact I24, and extending a circuit from the second auxiliary positive bus I I I, through the relay-contacts I23 and I24, to a fourth countingchain relay I25. When this last-mentioned relay is energized, on the second restoration of linevoltage, it completes a holding circuit, through its front-contact I26, around the relay-contacts I23 and I24, and it also interposes a second break in the energizing circuit of the vibrating relay I92, this second break being introduced by means of back-contacts I21 carried by the relay I25. The fourth counting-chain relay I25 also has front-contacts I28 which are disposed in series with front-contacts I28 on the counter-controlling relay I80.

If the voltage dips a third time in both of the tie-lines I and 2, the counter-controlling relay IEIIi is again energized and it extends a circuit from the second auxiliary positive bus III, through the relay-contacts I28 and I29, to the operating coil of a fifth or final counting-chain relay I38, which has a delayed drop-cut action which is secured by a lag-ring I3 I, or other suitable means.

This last relay I30 of the counting chain has a front-contact I32 which serves to trip out the circuit breakers H and 22 in both of the tielines I and 2 in response to the third voltage-dip, this operation being accomplished by the energization of a tripping relay I33, the energizing circuit of which is completed through a circuit extending from the auxiliary positive bus 59, through an out-of-step operation-indicator I36, thenceto the front-contact I32 of the relay I32, and thence to the energizing circuit I35 of the tripping relay I33. The tripping relay I33 has two front-contacts I33 and I3? which close the tripping circuits of. the breakers 2! and 22, respectively.

The last relay I32 of the counting chain has a holding circuit which is provided by a front-contact I38 which by-passes the relay-contacts I28 and I29, which insures positive action notwithstanding chattering or imperfect contacts in either one of the relay contacts I28 or I29.

The last relay I38 of the counting chain also has a back-contact I39 which disconnects the third auxiliary positive bus II2 from the second auxiliary positive bus III, thus releasing the relays H3, H5 and I25 which no longer need to be held closed and which thus constitute an unnecessary drain on the relaying buses. The deenergization of the relay IIB closes the back-contact III! of the latter, but this does not effect the reenergization of the vibrating relay I02, because of the interruption of this circuit at the backcontact I2'I of the relay I25. The vibrating relay thus continues to vibrate until it finishes its set period of vibration.

It will thus be noted that, at the first restoration of the line-voltage after the first voltagedip, the vibrating relay I32 starts its set course of vibration and continues to do so for its allotted period of 3 to 5 seconds, or any other predetermined time. Starting with the first voltage-dip, the counting-chain relays II3, II6, I2I, I25, and I32 begin operating, one after another, as normal line-voltage conditions are alternately restored and lost again, until, on the third voltage-dip, the

last counting-chain relay I32 is energized and actuates the tripping relay I33 which opens the line circuit-breakers EI and 22, provided that all of this operation takes place before the vibrating relay I22 stops vibrating. if this complete chain of operations is not completed within that time, the timer relay Iil'I drops out opening its backcontact 562 and thus deenergizing any one of the counting-chain relays II3, I I 5, I2I, and I25 that may have been energized meanwhile, so that the apparatus is then in condition for a new operation just as if nothing had happened. In this Way, We are enabled to discriminate between a single interruption and restoration of voltage which might result from a number of causes other than out-of-synchronisrn operation, and a continuance of this cycle for any predetermined number of times within a given period, depending upon the number of relays that we use, in accordance with our counting-chain and our timing elements I22 and IN. This takes care of the fast drifting out-of-step of the two ends of the tie-lines I and 2.

In cases where the trolley-conductor circuit is normally connected together across the circuitinterrupting point of the transmission lines I and as is done in the illustrated case by means of the trolley breaker 23, it is necessary to open the trolley breaker when the line breakers 2I and 22 are opened, and this may be conveniently accomplished by back-contacts MI and I22 on the line breakers 2i and 22, which are utilized to complete a tripping circuit to the trolley breaker 23.

The element used for sectionalizing the tie-line I, 2, in the event of slow pull-apart, is an over current relay I 33 (Fig. 4), so connected as to totalize the current in all of the lines, or practically all of the lines, connecting the two supply points L and R (Fig. 1). This over-current relay M3 is energized cumulatively from current transformers MCI and I25 connected in lines I and 2 at station D. In order to allow for occasional brief power-exchange conditions under which extremely high load-currents may be transmitted by the tie-lines I, 2, and in order to avoid the necessity for utilizing a relay-settng high enough to avoid these extreme power-exchange currents, thereby running into the difficulty of not responding to out-of-synohronism currents when there is only a small total generator-capacity on the sysj lines I and 2.

The operating coil of the short-circuiting relay I46 is energized by means of a front-contact I5I on the auxiliary voltage relay M which responds to the voltage on the line I. The operating coil of the short-circuiting relay I ll is energized by means of a front-contact I52 on the auxiliary voltage relay 92 which is energized whenever there is normal voltage on'the line 2. By normal voltage, we mean voltage such as is tolerated "on the trolley circuit, which may be as low as 50 or 60 percent of the rated voltage.

With the particular connections shown in 4, the over-current relay I33 is normally ShOlt-r circuited by both of the short-circuiting relays I45 and M7. If the voltage drops below the relay-settings of the voltage dip relays 8i and 82 on both of the tie-lines I and 2, these short-circuiting relays I56 and III? become deenergized and remove the short-circuit from the current winding of the over-current relay M3, permitting the latter to function if there is a sufiicient overcurrent at the same time.

The over-current relay M3 has front-contacts I53 which serve to make a connection from the positive relaying bus to the energizing circuit I35 of the tripping relay I33, thereby tripping both of the line circuit-breakers 2| and 22.

rected by the primary protection means 26 at either generating station L or R. In other words, sufiicient time should be allowed to permit the diiferential current relay 26 to operate, and sumcient time after that to permit the circuit breaker 24 or 25 to open its contacts and cease arcing, and time for opening any step-down-transformer breakers I55 or any other breakers necessary to clear the faulty line, before the over-current relay M3 at the intermediate station D shall close its tripping contact I53. This necessitates a certain minimum time-setting below which it is not desirable to set the over-current relay I 33.

At the same time, it is desirable to have the over-current tie-interrupting relay I63 operate before the secondary protection means II, 52, at either one of the generating stations L or R so that the generating stations will not be disconnected from both lines I and 2 in response to the the corresponding current on the secondary protection over-current relays II and 42 at the generating station L.

Referring back to Fig. 2, it will be noted that we have indicated, by a dotted line I56, the voltage-relay setting of the fault-detector or voltage-dip relays BI and 82 (Fig. 4). The overcurrent relay I43 (Fig. 4) is not energized except when the voltage is below the setting I56 in Fig. 2, so that it will be seen that, during out-ofsynchronism conditions, the over-current relay I43 will be energized only during the peaks of the current surges, as indicatedby the shaded areas in Fig. 3. 'If the speed at which the two 25-cycle generating stations L and R pull-apart is so great that the duration of one of the shaded areas in Fig. 3, during which the over-current relay I23 is energized, is less than the timesetting of the relay, which is 1.5 seconds in the illustration given, this over-current relay will not pick up during the first over-current period of come re-set, or practically re-set, before the sec- 0nd shaded area in Fig. 3, so that it might never pick up. Thus, we have provided the first-described out-of-synchronism relaying apparatus utilizing our counting-chain for responding to fast pull apart.

Our over-current relay I43, for responding to slow pull-apart of the two 25-cycle generating stations, is reliably operative to respond to prolonged overcurrent'pulsations which occur during slow pull-apart, when the duration of each shaded area in Fig. 3 is commensurate with, or greater than, the time-setting of the'over-current relay I43 or when the over-current relay will operate in a few dips. The over-current relay I 33 trips out the tie-interrupting breakers 2I and 22 in response to the over-current swing of the slow out-of-synchronism movement, thereby afiording protection under circumstances when the voltage cycle-counter would not 'give protection at all, unless the overall time-settings were made rather undesirably long. I

In the particular railroad electrification system illustrated in the drawings, there were conditions of operation, when. other interconnections were made, beyond those shown in Fig. 1, whereby the primary protective means in some stations required more time for tripping, thus necessitating an over-current relay-setting longer than 1.5 secondsat the intermediate sectionalizstations. This constituted only a rare emergency r operating-condition, which was taken care of, in the apparatus shown in Fig. 4, by utilizing a second over-current relay I65 at the sectionalizing sub-station D, said relay having afront-contact I BI which closes, to energize the tripping relay I 33, after a long time-setting of, said 4. seconds, as indicated by the heavy dashpot I62 in Fig. 4.

The operating coils of the two over-current relays I23 and I59 are connected in series across the current-responsive relay-supply circuit. The operating coil of the long-time over-current relay IISEl is normally short-circuited by the ,energization of a relay I63 which is controlled by a doublethrow push-button I64. When the push-button IE4 is in one position, it energizes this relay I63.

and short-circuits the current-coil of the long time over-current relay I66. When the push button I65 is in the other position, it energizes another relay I65 which has a front-contact I66 for short-circuiting the current-coil of the shorttime' over-current relay I43. The relay I65 also has a back-contact IIS'I which disconnects the auxiliary positive bus 99 from the positive terminal of the relay supply source, so as to render the automatic voltage-dip counter inoperative under these conditions.

Our invention also provides a safeguard against a system-disturbance, such as regulatortrouble in one of the generator-stations, or extreme overload conditions in only one section of the system, but resulting in a general depression of the trolley-voltage to a point below which the rotating main or auxiliary tractive machines of the locomotives (not shown) will not operate properly. When such low-voltage conditions obtain on the trolley conductors at the sectionalizing station D, it is desired to take cognizance of that fact, and if said conditions persist for a predetermined time, to segregate the two sections of the electrification-system at that point, so that operable voltage-conditions may be maintained in at least half of the system, rather than putting the entire system out of service until the trouble can be corrected. There may even be a possibility that out-of-synchronism conditions might eventually be brought about by such a low-voltage condition, which constitutes another reason for segregating the faulty section from the good section when the former is so bad that the good section is no longer satisfactorily operative.

Accordingly, as shown in Fig. 4, we have provided an undervoltage relay I16, energized from a potential transformer! ll connected to one of the trolley conductors 3 at the sectionalizing station D. When the undervoltage relay I'IIJ drops, its back contact I72 energizes a timing-relay ITI3, which, after a regulable, predetermined time, closes its contact I l6 andthus completes a circuit from the positive relaying bus, through said contact I74, to an operation-indicator I75, and thence to the energizing-circuit I35 of the tripping relay I33, which, as previously described, opens the line circuit-breakers 2i and 22, and through themthe trolley breaker 23. The trolley-voltage timer I13 be of any suitable type, being indicated very diagrammatically as having both a lag-ring H6 and a heavy dash-pot I TI for securing the desired time-delay, but any other type of timer may be utilized.

While Fig. 4 shows two out-of-synchronism detectors, one designed more particularly for fast pull-apart, and the other designed more particularly for slow-pull-apart, it is to be understood that either protective means may be utilized alone, where it is felt that either one may be made to cover the expected range of speeds of pull-apart.

The overcurrent protective mechanism for detecting out-of-synchronism conditions possesses the advantages of adjustable current-settings and adjustable time-settings which provide selectivity with respect to other sectionalizing points, rendering this mechanism more advantageous, in these respects, than the counting-chain system.

An important advantage of the counting chain is-its ability to operate to sectionalize the system in the event of rapid pull-apart, and in its utilization of a counted number of alternate voltagedips and voltage-restorations, thereby providing a sure discrimination from fault conditions which do not result in loss of synchronism.

Figs. 5 and 6 show modifications of the overcurrent out-of-step detector, which may be substituted for the corresponding detector in Fig. 4, or which may be utilized alone, without the counting-chain detector.

In Fig. 5, the sectionalizing breaker 2! in line I is directly tripped by a front-contact I88 of an overcurrent relay I85 which is similar to the overcurrent relay ofFig. 4. It is energized from a line-current transformer I82, and is provided with adashpot I83 similar to the corresponding dashpot I54 in Fig. 4. There is provided a fault detector inthe shape of an undervoltage relay i8 1 which is energized from a potential transformer E85 so that normal line-voltage causes the relay to pick -up, opening its back-contact I86.

The improvement introduced in Fig. 5 consists in the incorporation of means forcausing a voltage-dip to quickly remove a short-circuit on the current-transformer I82, but arranged so that a voltage-restoration will not remove said short-circuit for a predetermined time. These results are obtained by means of a timer-relay I81 which is set in operation by a closure of the back-contacts I86 on the undervoltage relay, and which opens its back-contacts I88 quickly, upon the energization of the timer-relay I8I. Upon deenergization of the timer-relay I8'I,-the backcontacts !88 close, but only after a predetermined time which is controlled by a dashpot I 89 or other timing means. The contacts 588 are connected across the operating coil of the overcurrent-relay I8I.

The effect of the timerI8'I will be'better-understood upon reference to Figs. 2 and-3. In

these figures, the time-lag in the opening and closure of the fault-detector contacts is ignored. which is permissible if the :time represented by Figs. 2 and 3, on the X-axis is large. But'if the frequency of the out-of-synchronism pulsations increases, as in very fast pull-apart, the total time represented by Figs. 2 and 3 becomesmuch smaller, and hence the lag of the fault-detector will become an appreciable part of the total time. This will result in the shifting, to the right, of both the beginning and the ending of each shaded area in Fig. 3, and such shifting may be small or great, and may have different values for the beginning and ending, respectively. If this rightward shifting is large, there will be a tendency for the voltage relay to remain permanently energized or deenergized, depending on whether the average voltage is greater or less than the relaysetting.

These difficulties in the response to the rootmean-square voltage, by the voltage relay, during rapid pulsations in the out-of-synchronism power, are largely eliminated by the introduction of the timer-relay IS'I in Fig. 5. With the aid of this relay I87 it is possible to cause the combined effect of the undervoltage relay i8 3 and the timerrelay I81, which together constitute the faultdetector, to be, that the overcurrent relay I8I will be energized promptly upon the occurrence of the first voltage dip to a value indicated by the relay-setting E5.) in Fig. 2, and the overcurrent relay will thereafter remain continuously, or substantially continuously, in service during even fairly rapid pull-apart, so that the time-setting of the overcurrent relay, as determined by the dashpot I83 may be independent of the duration of each individual voltage-dip.

It is desirable that the voltage-responsive relay IM (Fig. 5) shall be designed to respond as rapidly as possible to integrated power, or average or root-mean-square values of voltage, without responding to individual cycles of the linefrequency voltage-wave. It will be remembered that Fig. 2 shows integrated or root-meansquare voltage, so that the cycles shown are not line-frequency cycles of the alternating voltagewave, but much slower cycles of the pulsationssin root-mean-square voltage due to out-of-synchronism conditions.

We are. not limited tomechanical relays, as 'distinguished from tube-relays, or to the use of a fixed, predetermined time in the delay response of the fault detector to a restoration of voltage after a voltage-dip. Both of these variations are illustrated in Fig. 6.

In Fig. 6, the voltage-dip-response is obtained bymeans of a tube I99, controlling an auxiliary V anode I95, a cold, large-area cathode I96, and

a control-electrode called a grid I91, usually located quite close to the anode. When the grid gets sufficiently positive with respect to the cathode, even for a very brief moment of time, a glowdischarge starts in the tube and instantly spreads over to theanode, whereafter the anode-cathode discharge commences, and thereafter continues independently of the grid.

The-grid-glow tubes are controlled in response to the line-voltage by means of a potential transformer 299, feeding through a'double-wave rectifier 29!, and aninductor 292, to a resistor 293. The size of the inductor necessary to properly smooth the rectified voltage-wave may be reducedby utilizing one or more filter-capacitors 209-. ,An intermediate point 295 of the resistor 293 is connected to the positive bus of the station relay supply circuit, as are also the two anodes I95 of the tubes I 99 and I92. Other resistor-taps 295 and 29?, on opposite sides of the intermediate point 295, are connectedto the grids I9l,of the respective tubes I99 and I92 through current-limiting resistors 298 and 299.

The voltage-dip-responsive tube I99 has its anode I95 connected to the positive supply-bus through a back-contact 2!! of the auxiliary restoration-responsive relay I93. The cathode I96 of the tube I99 is connected to the negative supply bus through the operating coil of the auxiliary voltage-dip relay !9!, and an auxiliary breaker-switch 2I3 which opens when the'circuit-breaker 2! opens.

The voltage-restoration-responsive tube I92 has its anode I95 connected to the positive supply-bus through the operating coil of the auxiliary restoration-responsive relay I93, while its cathode I96 is connected to an intermediate negative voltage obtained by means of a potentiometer 2 I5 which is connected across the positive and negative buses and with the breaker-switch 2I3 interposed in the negativebus connection. Between the potentiometer 2I5 and the cathode I96 of the restoration responsive tube I 92 is interposed a front contact 2i! of the auxiliary voltage-dip relay I9I. It is possible to utilize tube relays also'for the current-responsive element, as well as for the voltage-responsive elements. In Fig. 6, however, a mechanical-type overcurrent relay I8! is shown, the same as in Fig. 5, and its terminals are normally short-circuited by a back-contact M9 on the auxiliary voltage-dip relay NH.

The operation of the system shown in Fig. 6 will best be understood by reference to the curves of Figs. 7 and 8. The rectified voltage-wave is shown at 229 in Fig. '7. This is the voltage that would be impressed on the resistor 203 if there were no inductance in'thecircuit. The average value of this rectified voltage-wave is indicated at 22! in Fig. 7. This is the voltage that would be impressed on the resistor 293 if the inductor 292 had an infinite value, or if the filter 294 worked perfectly. The actual voltage impressed on the resistor 293 has an intermediate value, as indicated by the dotted wavy line 222 in Fig. 7.

Fig. 7 shows a voltage-dip in the line-voltage.

At a certain drop in voltage, as indicated by the value 223, it is desirable to cause the voltage-dip tube I99 to become conducting, and this action is obtained when the partially smoothed wave 222 crosses the critical value 223. (It will be noted that Fig. 7 is drawn as if the full rectifier output were applied to the anode-grid circuit of the tube I99, instead of only the portion included between the taps 295 and 299, which merely involves a change of scale, as will be obvious to those skilled in the art.) When the line-voltage rises again to some higher value, as indicated at 229 in Fig. '7, it is desired to cause the restoration-responsive tube I92 to become conducting, and this action is obtained when the partially smoothed wave 222 crosses the critical value 229.

Fig. 8 shows the anode and grid potentials 225 and 229 of the voltage-dip tube I99, and the anode and grid potentials 227 and 228 of the voltage-restoration tube I92, all plotted with reference to the cathode-potentials of the respective tubes. As a matter of fact, the anodes of the two tubes are both at the same potentials while the cathodes are at different potentials, but in Fig. 8the cathode-potential of each tube has been taken as the datum-line to which all other potentials of that tube are referred.

In the voltage-dip-responsive tube I99, the

grid-cathodepotential 22 6 is less than the anodecathode potential 225. When the line-voltage decreases, the grid-cathode voltage 229 increases,

approaching the anode-cathode voltage 225, and

at a certain critical value 229 the glow-discharge starts, picking up the auxiliary voltage-dip relay I9! (Fig. 6). This removes the short-circuit from the overcurrent relay I8! and also applies voltage to the cathode I95 of the second tube I92 which responds to a restoration of the line voltage.

Ifthe line-current is above the setting of the overcurrent relay I8! (Fig. 6), the latter begins to operate, in a time determined by thedashpot I83.

When the line-voltage begins to come back to normal, after a dip, the grid-cathode potential 228 (Fig. 8) of the second tube I92 rises, because the voltage tapped off of the resistor 293 (Fig. 6) adds on to the anode-cathode potential 221 (Fig. 8). At a predetermined critical value 239, Fig. 8, the second tube I92 becomes conductive, picking up its auxiliary relay I93 and opening the back,- contact 2!! of the auxiliary relay after a timedelay determined by the dashpot I99, if the dashpot is used. When the back-contact 2! I is opened, the anode-cathode circuit of the first tube I99 is opened, thereby releasing the auxiliary voltagedip relay I9! which re-opens the anode-cathode circuit of the voltage-restoration tube I92 and at the same time re-applies the short-circuit to the operating-coil of the overcurrent relay IBI. This restores the apparatus to its original normal condition, ready to respond, as previously described, to another voltage-dip, or, if the'time delay introduced by the dashpot I94 is long enough for a second voltage-dip to be already under way, the voltage-dip tube I will immediately again spill over, instantly removing the short-circuit from the overcurrent relay l8l before the latter has had time to reset. It will be noted that the dashpot 194 does not delay the dropping-out action of the auxiliary voltage-restoration relay While we have shown our invention in several preferred forms of embodiment, it will be obvious that many changes in the arrangements of the voltage-dip counter and other details of the connections of the various other parts may be resorted to without departing from the essential spirit and theory of our teachings. We desire, therefore, that the appended claims shall be accorded the broadest interpretation consistent with their language and the prior art.

We claim as our invention:

1. Out-of-synchronism responsive means for an electric tie-line of a synchronous alternatingcurrent transmission system, said system including means for, at times, interrupting the circuit of said tie-line, said out-of-synchronisrn responsive means including, in combination, means for counting and responding to a plurality of cycles of a predetermined drop and restoration of tie-line voltage within a predetermined time, and means for responding to a sustained overcurrent condition that persists for a predetermined time.

2. Protective means for a synchronous alternating current system comprising a plurality of generator stations, a tie-line connected therebctween, primary and secondary protection means for controlling the power-supply from each generating station to the tie-line and for disconnecting the power-supply under abnormal conditions in connection with said power-supply from said generating stations to said tie-line, the primary protection means acting promptly and being selectively responsive to predetermined fault conditions, the secondary protection means acting only after a time-delay and being less selective in its response to abnormal conditions, seetionalizing circuit-interrupter means disposed at an intermediate point in said tie-line, and sectionalizing protective means. for automatically controlling said sectionalizing circuit-interrupter means, said sectionalizing protective means being responsive to abnormal tie-line conditions and acting in a time between the times of said primary and secondary protection means.

3. Protective means for a synchronous alternating-current transmission system comprising a plurality of generator stations, 2. tie-line connected .therebetween, primary and secondary protection means for controlling the power-supply from each generating station to the tie-line and for disconnecting the power-supply under abnormal conditions in connection with said powersupply from said generating stations to said tieline,-the primary protection means acting promptly and being selectively responsive to predetermined fault conditions, the secondary protection means acting only after a time-delay and being less selective in its response to abnormal conditions, sectionalizing circuit-interrupter means disposed at an intermediate point insaid tieline, and sectionalizing protective means for automatically controlling said sectionalizing circuitinterrupter means, said sectionalizing protective means including overcurrent means operative in atimebetween the times of said primary and secondary protection means;

4. Protective means for a synchronous alternating-current transmission system comprising a plurality of generator stations, a tie-line conneoted therebetween, primary and secondary protection means for controlling the power-supply from each generating station to the tie-line and for disconnecting the power-supply under abnormal conditions in connection with said powersupply from said generating stations to said tieline,-the primary protection means acting promptly and being selectively responsive to predetermined fault conditions, the secondary protection means acting only after a time-delay and being less selective in its response to abnormal conditions, sectionalizing circuit-interrupter means disposed at an intermediate point in said tieline, and sectionalizing protective means for automatically controlling said sectionalizing circuitinter-rupter means, said sectionalizing protective means including overcurrent means operative in a time between the times of said primary and secondary protection means, and means for normally deenergizing said overcurrent means and for energizing the same only in the event of abnormal tie-line conditions indicative of faulty operative conditions.

5. Protective means for a synchronous alternating-current transmission system comprising a plurality of generator stations, 2. tie-line connectedtherebetween, primary and secondary protection means for controlling the power-supply from each generating station to the tie-line and for disconnecting the power-supply under abnormal conditions in connection with said powersupply from said generating stations to said tieline, the primary protection means acting promtply and being selectively responsive to predetermined fault conditions, the secondary protection means acting only after a time-delay and being less selective in its response to abnormal conditions, sectionalizing circuit-interrupter means disposed at an intermediate point in said tie-line, and sectionalizing protective means for automatically controlling said sectionalizing circuit-interrupting means, said sectionalizing protective means including means for counting and responding to a plurality of cycles of a predetermined drop and restoration of tie-line voltage within a predetermined time.

6. Out-of-sychronism responsive means for an electric tie-line of a synchronous alternatingcurrent transmission system, said system including means for, at times, interrupting the circuit of said tie-line, said out-of-sychronism responsive means including means for counting and responding to a plurality of cycles of a predetermined drop and restoration of tie-line voltage within a predetermined time.

7. Out-of-sychronism responsive means for an electric tie-line of a synchronous alternatingcurrent transmission system, said system includ ing means for, at times, interrupting the circuit of said tie-line, said out-of-synchronism responsive means comprising a plurality of generating stations having synchronous machines at opposite ends of said tie-line, said out-oi-synchronism cyclic response, and means so controlling said' sponse to the counting of said; predetermined number of cycles Within a predetermined time.

8.'Protective means fora synchronous alternating-current system comprising a plurality of generator stations including synchronous machines, a tie-line connected therebetween, sectionalizing circuit-interrupter means disposed at an intermediate point in said tie-line, and sectionalizing protective means for automatically controlling said sectionalizing circuit-interrupter means to efiect a sectionalizing operation in said tie-line, said sectionalizing protective means including means for responding to a cyclic change of electrical conditions in said tie-line, indicative of the c. possible slipping of the poles of a synchronous machine at onegenerating station with respect to a synchronous machine at the other generating station, means for counting a plurality of such cycles up to a predetermined number, timing sectionalizing circuit-interrupter means as to efect said sectionalizing operation in response to the counting of said predetermined number of cycles Within a predetermined time.

9. Out-of-synchronism responsive means for an electric tie-line of a synchronous alternatingcurrent transmission system, said system including means for, at times, interrupting the circuit of said tie-line, said out-of-synchronism responsive means including a voltage-dip relay'for responding to a predetermined drop and restoration of a relaying voltage, means for deriving said relaying voltage from the tie-line, means for counting a plurality of operations of said voltagedip relay up to a predetermined number, timing means for responding to an operation of said voltage-dip relay, and means for effecting a predetermined operation in response to a completion of said predetermined number of operations of said voltage-dip relay within a predetermined time.

' 10. Out-of-synchronism responsive means for an electric tie-line of a synchronous alternating current transmission system, said tie-line comprising a multi-circuit transmission line constituting, in effect, a plurality of lines in parallel,

. said system including means-for, at times, interrupting the circuit of said tie-line, said out-ofsynchronism responsive means including means for counting and responding to a plurality of cycles of a predetermined drop and restoration of tie-line voltage within a predetermined time, and means for rendering said counting means inoperative'in full voltage remains on any one of the parallel circuits of said tie-line. 11. Out-of-synchronism responsive means for an electric tie-line of a synchronous alternatingcurrent transmission system, said tie-line comprising a multi-circuit transmission line constituting, in effect, a plurality of lines in parallel, said system including means for, at times, interrupting the circuit of said tie-line, said out-ofsynchronism responsive means including means for responding to a sustained overcurrentcondition that persists in the protected tie-line for a predetermined time, and means for normally deenergizing said overcurrent means and for, energizing the same only in the event of a simultaneous loss of voltage, to at least a predetermined extent, in each one of the parallel circuits of said tie-line. V M U I:

12. Out-of-synchronism responsive means for an electric tie-line of a synchronous alternatinga aa e current transmissionsystem, said system includ-v ing means for, at times, interrupting; the circuit of said tie-line, said out-of-synchronism responsive means including, in combination, meansfor counting and responding to a plurality of cycles of a predetermined drop and restoration of tie line voltage within 'a predetermined time, means for responding to a sustained over-current condition that persists for a predetermined time, and means for responding to a sustained under-voltage condition that persists for a predetermined time.

13. An electric-power system comprising two conductor sections and interconnection means normally disposedctherebetween, in combination with means responsive to a predetermined dip in conductor-voltage for energizing a timer, said timer operating, if its energization is maintained for a predetermined time, to efiect a predeter mined circuit-operation, and means responsive to said circuit-operation for interrupting said interconnection. c 7

.14. Out-of-synchronism responsive means for an electric tie-line of a synchronous alternatingcurrent transmission system, said system including means for, at times, interrupting the circuit of said tie-line, saidout-of-synchronism responsive means including slowly acting overcurrent means responsive to a sustained over-current condition that persists for a predetermined time, 5 l

dition that persists for a predetermined time, j

means for normally deenergizing said overcurrent means, means quickly responsive to a drop of'the tie-line voltage to a predetermined subnormal value, for efiecting the energization of said over- 7 current means, and means responsive to an increase of the tie-line voltage to a second predetermined subnormal value greater than the firstmention'ed subnormal value, for deenergizing said overcurrent means.

16. Out-of-synchronism responsive means for an electric tie-line of a synchronous alternatingcurrent transmission system, said system inclnd-i ing means for, at times, interrupting the circuit of said tie-line, said out-oi-synchronismresponsive means including slowly acting overcurrent means responsive to a sustained over-current condition that persists for a predetermined time, means responsive to .a sustained overcurrent means, means quickly responsive to a predetermined dip in tie-line voltage for efiecting the energization of said overcurrent means, and timedelay means for assuring the continuance of the energization of said overcurrent means for a predetermined time afterit is once putin operation by said voltage-dip-responsive means.

17. Protective means for an electrical system comprising a generator station and a pair of transmission lines connectedthereto, primary and secondary protection means for controlling the power-supply from the generator. station to each transmission line and for disconnecting. the power-supply under abnormal conditions, the primary protection means acting promptly and being differentially and selectively responsive to predetermined fault conditions, said primary protection means comprising a differential relay for comparing conditions in the pair of transmission lines, and the secondary protection means acting, in general, only after a time delay and being less selective in its response to abnormal conditions, said secondary protection means comprising an individual fault-responsive relay for each of the protected lines, means for normally deenergizing both the primary and the secondary protection means, and fault-detector means for energizing the primary protection relay in the event of an abnormal voltage-dip on either transmission line and for energizing the individual secondary protection relay only in the event of a voltage-dip on its own transmission line.

18. Protective means for an electrical system comprising a generator-station bus and a transmission line normally connected thereto, transmission-line protection means for controlling the power-supply from said generator-station bus to said transmission line and for disconnecting said power-supply under abnormal conditions, a plurality of generators adapted for connection, from time to time, to said generator-station bus, a plurality of generator circuit-breakers interposed between said generators and said bus for connecting and disconnecting the several generators to and from the bus, and recalibrating means directly responsive to the number of generator cirsuit-breakers open or closed for automatically adjusting a setting of said transmission-line protective means in a substantially definitely predetermined relation to the connected generator capacity at all times, said recalibrating means including a pilot-motor, a differentially excited electromagnetic reversing-switch for controlling said motor, means for disturbing the balance of said electromagnetic reversing-switch in response to an operation of a generator circuit-breaker, and means for restoring said balance in response to the operation of the pilot-motor.

19. Protective means for an electrical system comprising a generator station and a pair of transmission lines connected thereto, primary and secondary protection means for controlling the power-supply from the generator station to each transmission line and for disconnecting the power-supply under abnormal conditions, the primary protection means acting promptly and being differentially and selectively responsive to predetermined fault conditions, said primary protection means comprising a difierential relay for comparing conditions in the pair of transmission lines, and the secondary protection means acting, in general, only after a time delay and being less selective in its response to abnormal conditions, said secondary protection means comprising an individual fault-responsive relay for each of the protected lines, said primary and secondary protection means comprising serially connected current-responsive windings, variable transformer means whereby the strength of current in said secondary protection means may be varied with respect to that in said primary protection means, said generator station comprising a generator-station bus and a plurality of generators adapted for connection, from time to time, to said generator-station bus, a plurality of generator circuit-breakers interposed between said generators and said bus for connecting and disconnecting the several generators to and from the bus, and recalibrating means directly responsive to the number of generator circuitbreakers open or closed for automatically adjusting said variable transformer means.

MAX J. RUBEL. EDWIN L. HARDER. 

